1. Field of the Invention
The present invention generally relates to electrostatic discharge protection (ESD) and, more particularly, to a hot-wire tungsten resistor used as a current limiter to protect circuits from ESD damage.
2. Description of the Related Art
As electronic components are getting smaller and smaller along with the internal structures in integrated circuits, it is getting easier to either completely destroy or otherwise impair electronic components. In particular, many integrated circuits are highly susceptible to damage from the discharge of static electricity, even at levels which can neither be seen nor felt. Electrostatic discharge(ESD)is the transfer of an electrostatic charge between bodies at different electrostatic potentials (voltages), caused by direct contact or induced by an electrostatic field. The discharge of static electricity, or ESD, has become a critical problem for the electronics industry. Device failures are not always immediately catastrophic. Often the device is only slightly weakened but is less able to withstand normal operating stresses and, hence, may result in a reliability problem.
The amount of static charge generated through the process of friction and separation will be influenced by the extent of the contact, the materials involved, relative humidity, and the texture of the materials. Static charges of up to 30,000 Volts are not uncommon and can be generated by the simple act of walking across a floor; yet a discharge of only 10 Volts can destroy a Class 0 ESD sensitive device. Thus much effort has been expended by industry to protect electronic devices from ESD damage.
In order to protect output drivers, ESD protection devices are often employed at the pads to shunt the ESD discharge away from the output circuitry. However, the breakdown voltage of the output drivers may be lower than the clamping voltage of the ESD circuitry. Therefore, an ESD generated current may flow through the output driver in parallel to the ESD circuitry. It is desirable to minimize the current in the driver and maximize the current in the ESD circuitry.
Adding a resistor in series with the output driver increases the voltage drop through the driver and forces more current to flow in the ESD circuit, however, this resistor cannot be made too large or it will decrease output current drive. A current limiting device that exhibits a low impedance at low current levels but a higher impedance at high current levels would be useful to limit ESD current in the driver without impairing circuit operation. Thus a device that has linear current-voltage (IV) characteristics at low currents, and non-linear IV characteristics at high current levels is desirable.
U.S. Pat. No. 5,051,860 to Lee et al. shows such a device comprising a resistor device in parallel with an ESD protection device. In a first resistance mode during normal operations, the resistor device acts to have a low resistance value and thus introduces virtually no additional load to the input/output (I/O) circuitry. In a second mode of operation, during an ESD discharge, the resistor has a second significantly higher resistance which reduces current values during the ESD event to thereby protect the I/O circuitry. However, the resistor device is fairly complicated requiring a lattice of connected semiconductor resistors, the current path through which is controlled by a plurality of transistors. This greatly adds to the overhead of the device and therefore a simpler resistor having similar characteristics is desirable.
It is therefore an object of the present invention to provide a simplified resistor that has linear IV characteristics at low currents, and non-linear IV characteristics at high current levels to protect electronic components from ESD events.
According to the invention, a tungsten wire resistor is placed in parallel with ESD protection circuitry and may be incorporated as a metalization layer on a chip. The width and length of the resistor is chosen so that the resistor experiences significant self-heating at the current level where current limiting and non-linearity is desired. At this current level, the resistor becomes hot and its resistance increases dramatically. As a result the voltage drop across it increases and excess current caused by the ESD event is diverted to the parallel connected ESD protection circuitry where it can be safely dissipated. The resistance and the voltage drop increases until the fusing point of the wire is reached. In this manner, the tungsten wire resistor has linear IV characteristics at low currents, and non-linear IV characteristics at high current levels to protect electronic components connected in series with the resistor during ESD events.